1. Field of the Invention
This invention relates to vehicle safety belt retractors and more particularly to those which permit withdrawal of the seat belt for buckling or to accommodate the body movements of the individual using the belt, but which lock to restrain the wearer when conditions require it in the interests of safety.
2. Description of the Prior Art
Many reel-type safety seat belt retractors are known which employ an inertial device for triggering a pawl into locking engagement with the ratchet teeth on the reel in response to neccelerated rotation of the reel in the unwind direction. Also different methods have been suggested for permitting the belt to be withdrawn for belting without activating the inertial device. This is commonly accomplished by designing the inertial device to respond only to relatively high rates of acceleration of the reel. This entails a compromise between sensitivity and convenience, often to the detriment of both.
Retractors have been suggested wherein the sensitivity of the triggering device gradually increases as the webbing -S withdrawn, being related to the amount of webbing extended. The suggested methods are usually complex, expensive to manufacture, and add to the possibility of retractor failure. Electrical and hydraulic methods of achieving a change in sensitivity related to the amount of webbing extended, are known. It is a characteristic of this arrangement, that the further the belt is withdrawn for buckling, the greater the chance that the locking mechanism will be inadvertently triggered. It also follows that the sensitivity would be greater for a stout person than for a slim person or child.
It is an object of this invention to provide a safety seat belt retractor in which the locking of the reel of webbing is triggered by an inertial sensing device having two levels of sensitivity, a low level which is in effect when the user withdrawn the webbing for buclking up, at which time the triggering of the locking mechanism would be an unneccessary annoyance; and a high level of sensitivity which prevails after the user is buckled up, and a ready response to any sudden accelerating motion of the reel is required to protect the user. It is also an object of this invention to achieve this bi-level sensitivity simply, without complicated and expensive additional structures.